1. The practice of sifting refuse from an office or technical
installation to extract confidential data, especially security-compromising
information (‘dumpster’ is an Americanism for what is elsewhere
called a skip). Back in AT&T's
monopoly days, before paper shredders became common office equipment, phone
phreaks (see phreaking) used to organize regular
dumpster runs against phone company plants and offices. Discarded and
damaged copies of AT&T internal manuals taught them much. The
technique is still rumored to be a favorite of crackers operating against
careless targets.

2. The practice of raiding the dumpsters behind buildings where
producers and/or consumers of high-tech equipment are located, with the
expectation (usually justified) of finding discarded but still-valuable
equipment to be nursed back to health in some hacker's den. Experienced
dumpster-divers not infrequently accumulate basements full of moldering
(but still potentially useful) cruft.